Contoured sidewalls for captured air bubble surface effect ships



United States Patent Robert M. Williams Fairfax, Virginia 733,052

May. 29, 1968 Oct. 6, 1970 Inventor Appl. No. Filed Patented Assignee the United States of America, as represented by the Secretary of the Navy CONTOURED SIDEWALLS FOR CAPTURED AIR BUBBLE SURFACE EFFECT SHIPS 4 Claims, 12 Drawing Figs.

US. Cl 1. 180/126, 180/1 16 Int. Cl 136% 1/04 Field of Search 180/126, 1 18 [56] References Cited UNITED STATES PATIENTS 3,066,753 12/1962 Hurley et a1 aI... 180/126X 3,182,739 '5/1965 Cockerell .1 180/118 3,21 1,247 10/1965 Rethorst 180/1 18 Primary Examiner-A. Harry Levy Attorney-L. A. Miller, Q. E. Hodges, A. Sopp and F. A.

Lukasik ABSTRACT: An arrangement for altering the wetted area of a sidewall captured air bubble surface effect ship to minimize frictional drag by contouring the sidewalls in such a manner as to provide only the required sidewall area necessary to prevent air leakage throughout the operating range of the craft.

v 0.010 1.9 FROUDE NUMBER =o.o1 0.5

STERN SEAL LIMITING P RE SS UR E I ENYEI OI EF INCHES Patented Oct. 6, 1970 Sheet 6 l6 l2 5 W I 8 3 4 0. 4 ML 14 I. m/ 0 VV 4 6 3 2 3 8 2 f 2 O E 2 m I s 16 T I. U l 0 INCHES FIG. 7?-

6 l6 5 2 5 4 8 M -4 O. 3 4 LL 4 O V W 4 6 3 2 3 8 2 4 2 0 E 2 m l s 6 T. l w W 8 4 INCHES FIG. 8.

OUTSIDE INSIDE zTIEmQ 3259 FIG. .9.

ATTORNEYS SIDEWALL DEPTH-IN. SIDEWALL DEPTH-IN.

SIDEWALL DEPTH- IN.

Patented Oct. 6, 1970 3,532,17

Sheet 5 of 4 4 OUTSIDE 2 INSIDE o l l l l I l I l I l l l I l I l l l l o 4 8 I2 I6 20 24 2e 32 3e 40 44 4 52 56 60 INCHES OUTSIDE 2 V INSIDE v O l l I I I I l l l l l l l l l I l l I I l l l I o 4 8 I2 I6 20 24 2s 32 3e 40 44 4a 52 56 so INCHES 6 v 0.0m L9 5 FROUDE NUMBER=0.0 Io 0.5

4 STERN SEAL O 4 8 I2 I6 20 24 28 32 36 4O 44 48 52 56 60 INCHES INVENTOR FIG" /2 ROBERT M." WILLIAMS ATTORNEY .S

CONTOURED SIDEWALLS FOR CAPTURED AIR BUBBLE SURFACE EFFECT SHIPS The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION The present invention relates generally to vehicles for traveling over water and which are supported, at least in part, by a cushion of pressurized air or gas contained beneath the vehicle, and more particularly pertains to contouring the sidewalls of a Captured Air Bubble (CAB) Surface Effect Ship (SES) to reduce frictional drag throughout the operating range of the vehicle.

Prior art sidewalls for CAB vehicles, have been of an essentially rectangular planform. The shape of the rectangular planform sidewalls increased frictional drag appreciably since a greater area of sidewall above what is actually required to prevent air leakage was being used The consequence of the rectangular planform is a waste of propulsive power, lower speed and increased operating costs. Prior to this invention data has not been available on the actual wave shapes in the cushion pressure region so that unnecessary sidewall area has been included as a safety factor to prevent venting.

SUMMARY The general purpose of this invention is to provide sidewalls for a CAB vehicle that have all of the advantages of present state-of-the-art sidewalls and have none of the above described disadvantages. To attain this, the present invention provides a unique sidewall contour which permits a sidewall craft to accelerate up to and maintain its cruise speed with a minimum of frictional drag while preventing air leakage under the sidewalls. The present invention contemplates an overwater vehicle wherein a pressurized air bubble is sealed between the vehicle and the water. The maximum air retention within the pressurized region between the vehicle and the water is maintained by a pair of thin sidewalls and a ski assembly fore and aft which together with the sidewalls afford a substantially airtight-watertight airspace between water and vehicle. The contoured Configuration of the sidewalls, minimizes the amount of air which must be resupplied to the pressurized area between the vehicle and the water to maintain the support of the vehicle.

It is an object of the present invention to provide an over water vehicle capable of operating efficiently at high speeds under open sea conditions.

Another object of the present invention is to provide and over water vehicle which has a very favorable lift-to-drag ratio at speeds which vary from low to high relative to the lift-todrag ratio of conventional watercraft.

Still another object of the present invention is to provide an over water vehicle which can operate efficiently at high length to beam ratios.

A further object of the present invention is to provide a captured air bubble vehicle with contoured sidewalls.

A still further object of the invention is to provide a captured air bubble over water vehicle utilizing sidewalls which are contoured to present an area sufficient to seal the air bubble.

BRIEF DESCRIPTION OF THE DRAWINGS The exact nature of this invention as well as other objects and advantages thereof will be readily apparent from consideration of the following specification relating to the annexed drawings in which:

FIGS. l ll show inside and outside sidewall wetted areas with a vehicle length to beam ratio of8.

FIG. 12 shows a sidewall contoured in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In order to assist in the understanding of frictional drag as well as the wave drag phenomena in the subhump region, intensive studies of the sidewall waveforms were undertaken. Data presented in Virginia Polytechnic Institute (VPI) report on Study of the Wavemaking of Captured Air Bubble Vehicles, Mar. 1968 by the inventor or other such information are utilized in the manner described below.

In general, at least two Wavemaking drag humps are present. The subhump drag is considerably larger than had been previously assumed. Contouring of the sidewalls with the use of interior profile charts will result in a minimum drag design. The results of tests on a length to beam ratio of 8 are shown in FIGS. 1-12.

The interior wave development of a CAB vehicle takes place as follows. Initially, a very small trough forms directly after the bow and then lengthens and deepens as the speed increases, forming a form of hydraulic jump. As the speed increases, wave breaking develops along the depressed region and gradually moves aft, diminishing in width as the water attaches to the sidewalls and trails rearward. At even higher speeds the wave breaking eases, the inflow is smooth and the waves emanating from the sidewalls extend back and intersect forming a three-dimensional peak. Throughout this speed range, the waveform under the stern seal is flat. The intersecting waves grow in magnitude creating substantial peaks, reflect off the sidewalls and cross and reflect several times depending on the particular length to beam ratio involved. Peaks form at the intersections while regions of concavity exist at the nodes on the sidewalls.

The development of the three-dimensional wave patterns at the rear seal indicates an area of concern for minimum drag design. With a conventional flat planing ski, two alternatives are open: (I) by lightly loading the rear ski, it will be permitted to plane on the crest of the stern wave thus creating minimum frictional and momentum drag, but requiring a substantial increase in fan power because ofthe air gaps; or (2) to load the ski more heavily, thus decreasing the air gap by cutting into the wave crest. In this case, the water impingement becomes large and there is a resultant increase of momentum and frictional drag.

The most practical result for subhump design is to contour the sidewalls. The objective of contouring is to use only enough sidewall to seal the air bubble, thus minimizing frictional drag. FIGS. 1-12 provide typical information on interi or wave profiles which are pertinent in determining the sidewall contours. The data provided in FIGS. 1-12 is used to establish the necessary planform contour as a function of four parameters: (1) length/beam ratio (L/B); (2) scale ratio; (3) specific cushion loading (PM/S); and (4) Froude number r g The following is an example of the steps which may be utilized to scale experimental data:

a. Establish prototype data;

Ratio of Length to Beam (L/B). l Ratio of cushion Pressure/square root of the decking planform area W/LXB PAC S :=wcight scaling parameter L B (2) where W=weight of the craft and L B=cushion planform area.

Maximum operating Froude number V/\/g I: (3) where V=Velocity in ft./sec. and g=32 ft./sec./sec. The

Froude number is the speed or velocity scaling parameter. Length of cushion area =L b. Select corresponding data curve, e.g. FIGS. lll with the above parameters or if interpolation is necessary, select the next highest and lowest curves.

c. Plot the interior waveforms from zero up to the required maximum Froude number. Draw the envelope as shown in FIG. 12.

d. Model dimensions may now be scaled directly by ratio:

length prototype/length model.

EXAMPLE In an example to illustrate the use of the above mentioned method the following may be used: Assume that the vehicle to be designed will have a gross weight of 5,000,000 pounds. (2500 tons) and will have a Length to Beam (L/B) ratio of 8. The amount of pressure required is determined by W/LXB.

Utilizing the information available in reports such as Dept. of Navy, David Taylor Model Basin, Aerodynamics Laboratory Research and Development Report 2334 entitled Performance Estimates of Captured Air Bubble Vehicles with Water Jet Propulsion" and the information as shown in FIGS. lll, it may be determined that a craft will operate efficiently at a P/VS of3.07.

Solving for B in Equation (2) we have Solving Equation (l0) for B with W=5,000 lbs.

5,000,000 177.0 B $6? (10 2.s2s Therefore. L=500 ft.

With the length of the prototype craft a known value of 500 feet, it is now possible to determine the scaling parameter which is:

Length of craft/Length of model (FIGS. ll 1) or In order to plot the interior wave form from zero up to the required Froude number, the maximum Froude number must be established. Since the Froude number depends on the operating speed of the craft, the design speed must be extablished. Setting an arbitrary value of V=64 ft./sec. and solving Equation (3) Referring now to FIG. 11, there is shown the highest Froude number at which the craft will operate. In order to establish the operating envelope, i.e. the sidewall contour, a composite of the interior wave forms of FIGS. [-11 is drawn and an envelope is provided to encompass the lowest points to which the interior wave forms extend as shown in FIG. I2. If, for example, a craft is to be designed at a particular speed and will normally only operate at this speed, then only one envelope contour is required. In the example above, at a velocity of 64 ft./sec. the Froude number of the example, which is .504, would fall slightly above the Froude number of .503 as shown in FIG. 11. The model speed of 1.9 meters per second is shown in FIG. 12 as L9 in the shaded area. The limiting pressure envelope as shown in dotted lines in FIG. 12, is the envelope of the maximum depressions at all speeds and the additional area of the rectangular model, the shaded area, is not needed to maintain the pressure seal. In order to reduce the pressure envelope to meaningful dimensions for constructing a model, the outlines shown may be drawn on a grid where they may conveniently be transferred to the model scale.

To convert the model dimensions to the prototype, full scale size, requires the multiplication of the height and length dimensions of the model by the scaling parameter or as in the example, by I07. I. The dimensions found on the pressure en velope constitute a scale drawing with the l:l07.l ratio.

If a specific cushion loading parameter (PVS) is not shown on the charts. e.g. FIGS. l-ll, it is necessary to interpolate linearly. For example, if it were desired to operate a vehicle at P/VS =2.6, it would be necessary to proceed with the analy sis above described to obtain a workable value (not shown), near the design value of 2.6. In this example, pressure envelopes have been established for a P/VS =2.04 (not shown). Subtracting the lower value 2.04 from the higher value 3.07=l.03. The difference between 2.04 and 2.5=.56. The

t .56 tesulting ratio 1.03

two known pressure envelopes and a line connecting the points would outline the desired pressure envelope.

In establishing a sidewall contour which will effectively contain the pressure in higher sea states, a slight additional sidewall depth of about 10 percent would be sufficient.

Thus a contoured sidewall which effectively reduces the wetted area and thereby reduces frictional drag of a captured air bubble vehicle has been disclosed.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings.

Iclaim: l. The method of contouring sidewalls for a CAB vehicle comprising the steps of:

establishing vehicle prototype data relating to the ratio of length to beam, the weight scaling parameter, the length of the cushion area, and the velocity scaling parameter for one specific velocity, from which to determine the parameters of a waveform present beneath the vehicle;

selecting from predetermined waveforms a specific waveform which corresponds to said parameters, said waveform being a graphic illustration of wave depth versus wave length for a specific prototype; and

contouring said sidewalls to conform to the shape of said waveform in accordance with the scale ratio, whereby minimum frictional drag is produced by a specific CAB operating at a specific velocity.

2. In a captured air bubble surface effect ship being of the kind which is at least partly supported above the surface of water by a cushion of pressurized fluid contained between fore and aft seals beneath the ship, the improvement comprising:

longitudinal, water-piercing sidewall means depending from the bottom of said ship on each side thereof, said sidewall means being constructed ofa rigid material and containing contoured bottom surfaces of predetermined shape, said contoured bottom surfaces being formed in accordance with the method of claim I.

3. The method of contouring sidewalls for a CAB vehicle comprising the steps of:

establishing vehicle prototype data relating to the ratio of length to beam, the weight scaling parameter, the length of the cushion area, and the velocity scaling parameter for various specific velocities from which to determine the parameters of waveforms present beneath the vehicle;

selecting from predetermined waveforms a plurality of specific waveforms, each of said specific waveforms corresponds to said parameters at a specific velocity, said waveforms being graphic illustrations of wave depth versus wave length for a specific prototype superimposing each of said specific waveforms;

constructing an envelope of said waveforms which minimally encompasses the lowest points to which each specific waveform extends; and

contouring said sidewalls to conform to the shape of said envelope in accordance with the scale ratio, whereby minimum frictional drag is produced by a specific CAB operating over a specific range of velocities.

4. In a captured air bubble surface effect ship being of the kind which is at least partly supported above the surface of is the interpolated ratio between the water by a cushion of pressurized fluid contained between fore and aft seals beneath the ship, the improvement comprising:

longitudinal, water-piercing sidewall means depending from the bottom of said ship on each side thereof, said sidewall 

